The L85 is far better in barrel length, being a bullpup and also better ergonomically designed as an all purpose weapon system.
In terms of power, they're equal as they both fire the 5.56x45 NATO round.
In terms of customisation, the M4 obviously wins seeing as the AR platform has been around since the 1950's. Plus the fact most Americans that own a firearm collection also own an AR hence there is a large market for them.
Apparently much more accurate and better range, as you'd expect. I don't think they're very easy to disassemble and maintain in comparison though? I don't know but obviously there's a lot of military guys on here who could tell you that.
I think it'd depend on the scenario too, in a close quarters environment, i'd have an AR15 whereas somewhere with a longer engagement distance, my choice may be different.
The L85 is significantly more reliable, more accurate, hits harder and reaches further, all thanks to the longer barrel increasing the muzzle velocity and the bullpup design making it easier to hold still due to more points of contact with the body.
However, I find the M4 a lot easier to manipulate and I think the weight of the L85, and the sights being mounted so far above the barrel really, really let it down in close range engagements.
I'm with Marcus, I find my L85 is a proper pig to use indoors, I tend to lean it against a wall and go for a pistol instead.
The weight and mag placement make it feel really restrictive with how you move around with it, and obviously being limited to right handed only is a huge disadvantage, one that I'm not sure better accuracy, range and reliability adequately overcome; considering the frequency significance of those pros will come in handy.
In terms of accuracy, with its longer barrel, (518mm compared to 370mm) the SA80 is far more accurate and powerful, its a simple matter of physics. In terms of reliability, the SA80 wins again. It's main drawback is that it is right handed only, however the Army now teaches the neat transition trick, as seen in this vid:
During battle field mission trials the SA80 was quite literally thousands of rounds ahead of any rival. Hence why H&K revamped the M4A1 in to the 416, based on what they learned.
Draw your own conclusion. Keep in mind also answer is less likely to be m4 if asked on a UK than on a US forum.